The present invention relates to method and apparatus for controlling the admission of steam to a steam turbine especially for cold start-up thereof, or steam turbine restart.
When steam turbines are stopped for short periods of times, the rotor body casing and shaft hold a temperature close to operational temperature, whereas the piping and fluid lines which have been closed or are remote will cool off rapidly. Therefore, at restart, there is a large temperature difference between the freshly admitted steam and the pipes and/or valves exposed to it. Also for cold start-up, the steam at high temperature and pressure enters the various organs leading to the turbine which are at a much lower temperature. These gradients of temperature cause thermal stresses which can be damaging to the equipment, in particular, the valves, the gland seal zone and the rotor and casing of the turbine. Since most often the boiler provides superheated steam, when the latter reaches the cold walls of the pipes and valves, it cools off and takes the form of wet steam. Condensation of water in the pipes, the valves and the gland seal region of the turbine is a cause of erosion which will damage the walls, seals and turbine blades.
The invention provides for a method of and apparatus for controlling the admission of steam to a steam turbine so as to prevent large differences in temperature along the critical passageways from the steam generator to the turbine, when the turbine is to be started, or restarted.
It is acknowledged as prior art in U.S. Pat. No. 4,091,450 (H. Bloch) to control steam throughput or steam temperature in order to limit thermal stresses on the organs of a turbine. This patent proposes for an optimal start-up to coordinate the start-up of the turbine and generator units by increasing simultaneously and steadily the load absorption of the turbine and the inlet temperature.
It is known from U.S. Pat. No. 3,959,973 (P. Meylan) to protect the shaft-stuffing boxes of a steam turbine from water condensation of the admitted steam by deriving the steam initially to the condenser until it has become dry and superheated.